The present invention provides a rust-preventive steel sheet, having corrosion resistance as well as excellent air-tightness after welding, as a steel sheet for automobile fuel tanks.
A fuel tank of an automobile is usually designed in accordance with the design of the body in the final stage, and the shape has tended to become more and more complicated in recent years. Moreover, since the fuel tank must be safe in an automobile, the material to be used for the fuel tank is required to have an extremely excellent deep drawability and not to crack, due to an impact, subsequent to forming. In addition to the requirements, it is also important that the material form a decreased amount of corrosion product leading to pitting corrosion and filter clogging, and that the material can be easily and stably welded.
A Pbxe2x80x94Sn alloy-plated steel sheet, which is called a terne steel sheet, (Kokoku (Japanese Examined Patent Publication) No. 57-61833) has heretofore been principally used as a material having such various properties. The steel sheet has chemical properties stabilized against gasoline, and shows excellent press formability due to the excellent lubricity of the plating. In addition to the Pbxe2x80x94Sn alloy-plated steel sheet, a Zn-plated steel sheet which is thickly chromated is also used. The steel sheet also has excellent formability and corrosion resistance though it is not as excellent as the Pbxe2x80x94Sn alloy-plated steel sheet. However, a material not using Pb is desired from the standpoint of decreasing environmental pollution.
One of the prospective fuel tank materials of automobiles in which Pb is not used is an Alxe2x80x94(Alxe2x80x94Si) plated steel sheet. Since Al forms a stabilized oxidized film on its surface, Al shows excellent resistance to corrosion caused by organic acids formed by the deterioration of alcohol, gasoline, etc. as well as to gasoline. However, there are several problems with using the Al-plated steel sheet as a fuel tank material. One of the problems is poor press formability. Since the Al-plated steel sheet has a very hard Fexe2x80x94Alxe2x80x94Si intermetallic compound layer (referred to as an alloy layer hereinafter) formed at the interface between the plating layer and the steel sheet, the material quality is lowered compared with that of a steel sheet having no Al-plated layer. The Al-plated steel sheet, therefore, tends to crack when severely formed.
The Al-plated steel sheet also has the disadvantage that the peeling of the plating and crack formation therein tend to take place from a starting point in the alloy layer. When cracks are formed in the plating, inner corrosion tends to proceed from the cracks, and pitting may result in a short period of time. Accordingly, corrosion resistance subsequent to forming is a serious problem.
Another problem is weldability. Although an Al-plated steel sheet may be resistance welded, the welding lacks stability to some degree. Moreover, the Al-plated steel sheet has a problem in that the weld zone shows poor air-tightness though the steel sheet may be subjected to resistance welding such as spot welding and seam welding. Although a fuel tank material is required to have air-tightness after welding so that the fuel neither leaks nor volatilizes, the Al-plated steel sheet has the problem that its weld zone tends to fracture when an internal pressure is applied after welding, and the steel sheet shows poor air-tightness after welding. This is a phenomenon which substantially does not take place in other plated steel sheets such as a terne steel sheet and a Zn-plated steel sheet but which appears significant in the Al-plated steel sheets alone. Though the reasons are not definite, it is thought that Al in the plating layer diffuses into the steel to exert undesirable effects.
The present invention solves the problems as mentioned above, and provides a new rust-preventive steel sheet for fuel tanks having excellent corrosion resistance without using Pb in an organic acid environment and excellent press formability with which the steel sheet may satisfy anticipated sever press conditions in the production process of the tanks, showing no poor air-tightness in the weld zone, and ensuring resistance to corrosion caused by an organic acid after forming.
Furthermore, the chemical composition of the steel is optimized in the present invention to ensure the air-tightness in the weld zone. Specifically, the properties of the steel sheet has been significantly improved by restricting the P content and adding B.
The present applicant has disclosed a hot-dip Al-plated steel sheet to which up to 30 ppm of B is added in Kokai (Japanese Unexamined Patent Publication) No. 60-165366, and a hot-dip Al-plated steel sheet to which 0.01% of B is added in Kokai (Japanese Unexamined Patent Publication) No. 60-103167. These inventions have intended to provide steel sheets having a high temperature strength or oxidation resistance at high temperatures, and B has been added to fulfill the object. Moreover, the steel sheets are naturally anticipated to be used for automobile exhaust system materials, etc. to be employed in a high temperature environment. In contrast to the inventions mentioned above, the present inventor has discovered that the optimization of the contents of P and B in the steel has significant effects on the improvement of the air-tightness after welding which is an essential property of fuel tank materials.
On the other hand, the present invention provides two methods for greatly improving the corrosion resistance of a steel sheet subsequent to forming, namely the other problem to be solved.
First, the present inventors have investigated the progress of corrosion in fuel tanks subsequent to forming, and found a corrosion behavior as described below. A principal corrosion component in the inner environment of fuel tanks is formic acid formed by decomposition of the fuel. Corrosion of the base steel begins from cracks in the plating and the alloy layer as starting points, and the corrosion proceeds at the interface between the base steel and the alloy layer, resulting in gradual floating of the plating from the base steel and entire corrosion. The corrosion proceeds at the interface between the base steel and the alloy layer because the potential of the alloy layer is nobler than that of the base steel in the presence of formic acid and consequently corrosion of the base steel near the alloy layer is promoted.
There are two methods for diminishing the corrosion based on such discoveries. One of the methods is to inhibit cracks in the alloy layer, and the other is to decrease the potential difference between the alloy layer and the base steel.
Accordingly, the present invention provides two methods as described below. One is based on the discovery that an Al-plated steel sheet having a high total elongation inhibits crack formation in the plating, and intends to inhibit the formation of cracks in the alloy layer by optimizing the chemical composition of the steel. The other one is based on the discovery that when the potential difference between the alloy layer and the base steel measured in an environment containing 100 ppm of formic acid and the balance water at 20xc2x0 C. is up to 0.35V, corrosion hardly proceeds, and tends to inhibit the progress of corrosion even when cracks are formed in the alloy layer by optimizing the chemical compositions of the steel and the plating. The potential of the alloy layer-base steel can be controlled by adjusting the chemical compositions of the steel and the plating bath, or by pre-plating before hot-dip plating. For example, the following procedures may be practiced: Cr is added to the steel; the steel surface is pre-plated with Cr; or a clad steel is used; and Zn, etc., is added to the plating bath.
That is, the aspects of the present invention are as described below.
(1) A rust-preventive steel sheet for fuel tanks excellent in air-tightness after welding and corrosion resistance subsequent to forming, which comprises
a steel sheet comprising, in terms of % by weight, up to 0.01% of C, up to 0.2% of Si, less than 0.6% of Mn, up to 0.04% of P, up to 0.1% of soluble Al, up to 0.01% of N, one or at least two of Ti and Nb in a total amount of at least the atomic equivalent of (C+N) and up to 0.2%, 0.0001 to 0.0030% of B, and the balance Fe and unavoidable impurities, and
a plating layer comprising 2 to 13% of Si in terms of % by weight, and the balance Al and unavoidable impurities on the surface of the steel sheet.
(2) A rust-preventive steel sheet for fuel tanks excellent in air-tightness after welding and corrosion resistance subsequent to forming, which comprises
a steel sheet comprising, in terms of % by weight, up to 0.01% of C, up to 0.2% of Si, less than 0.6% of Mn, up to 0.04% of P, up to 0.1% of soluble Al, up to 0.01% of N, one or at least two of Ti and Nb in a total amount of at least the atomic equivalent of (C+N) and up to 0.2%, 0.0003 to 0.0030% of B, and the balance Fe and unavoidable impurities, and
a plating layer comprising 2 to 13% of Si in terms of % by weight, and the balance Al and unavoidable impurities on the surface of the steel sheet.
(3) A rust-preventive steel sheet for fuel tanks excellent in air-tightness after welding and corrosion resistance subsequent to forming, which comprises
a steel sheet comprising, in terms of % by weight, up to 0.003% of C, up to 0.1% of Si, up to 0.4% of Mn, up to 0.02% of P, up to 0.1% of soluble Al, up to 0.01% of N, at least one of Ti and Nb in a total amount of at least the atomic equivalent of (C+N) and up to 0.2%, 0.0003 to 0.0030% of B, and the balance Fe and unavoidable impurities, and
a plating layer comprising 2 to 13% of Si in terms of % by weight, and the balance Al and unavoidable impurities on the surface of the steel sheet.
(4) A rust-preventive steel sheet for fuel tanks excellent in air-tightness after welding and corrosion resistance subsequent to forming, which comprises
a steel sheet comprising, in terms of % by weight, up to 0.003% of C, up to 0.03% of Si, up to 0.3% of Mn, up to 0.02% of P, up to 0.006% of soluble N, up to 0.1% of Ti, and the balance Fe and unavoidable impurities, and
a plating layer comprising 2 to 13% of Si in terms of % by weight, and the balance Al and unavoidable impurities on the surface of the steel sheet,
the steel sheet showing a total elongation of at least 45% after plating.
(5) The rust-preventive steel sheet for fuel tanks excellent in air-tightness after welding corrosion resistance subsequent to forming according to any one of (1) to (4), wherein the steel sheet comprises at least one element selected from the following group in the following amounts: 0.5 to 7% of Cr, 0.05 to 0.5% of Cu, 0.05 to 0.5% of Ni and 0.05 to 0.5% of Mo.
(6) The rust-preventive steel sheet for fuel tanks excellent in air-tightness after welding and corrosion resistance subsequent to forming according to any one of (1) to (5), wherein the amount of the Al plating layer is up to 50 g/m2 per side.
(7) A rust-preventive steel sheet for fuel tanks excellent in air-tightness after welding and corrosion resistance subsequent to forming, which comprises a steel sheet substrate for plating, an Alxe2x80x94Fexe2x80x94Si intermetallic compound layer thereon and a plating layer comprising Al and unavoidable impurities on the intermetallic compound layer, the difference between the immersion potential of the steel sheet substrate for plating and that of the intermetallic compound layer in a solution comprising 100 ppm of formic acid and the balance water and unavoidable impurities being up to 0.35 V.
(8) The rust-preventive steel sheet for fuel tanks excellent in air-tightness after welding and corrosion resistance subsequent to forming according to (7), wherein the Alxe2x80x94Si plating layer comprises 2 to 13% of Si, 0.5 to 5% in total of one or at least two elements selected from the group consisting of Sn, Zn, Sb and Bi, and the balance Al and unavoidable impurities.
(9) The rust-preventive steel sheet for fuel tanks excellent in air-tightness after welding and corrosion resistance subsequent to forming according to any one of (1) to (8), wherein the rust-preventive steel sheet comprises a chromate coating layer in an amount of 5 to 100 mg/m2 as Cr per side at least on one side of the Al plating layer.
(10) The rust-preventive steel sheet for fuel tanks excellent in air-tightness after welding and corrosion resistance subsequent to forming according to any one of (1) to (9), wherein the rust-preventive steel sheet comprises an organic resin coating layer on the top surface at least on one side thereof.
The present invention will be explained in detail. First, reasons for restricting the chemical composition of the steel will be explained.
C: In the present invention, the steel sheet must have such a good deep drawability that the steel sheet can be formed to have a complicated shape, for example, the shape of a fuel tank. In order to achieve the object, the steel sheet is preferred to have a C content as low as possible. Moreover, since the quality of the steel sheet is deteriorated by Al plating, the steel sheet is required to have a still lower C content. Since a predetermined formability cannot be obtained when the C content exceeds 0.01%, the upper limit of the C content is determined to be the above-mentioned value. However, when the shape of fuel tanks which will become more and more complicated in the future is considered, the C content is preferably up to 0.003%, more preferably up to 0.0018%.
Si: Si has strong affinity with oxygen, and tends to form a stabilized oxidized film on the Al plating surface in the hot-dip Al plating step. When the oxidized film is formed, the oxidized film hinders an Al-Fe reaction in the plating bath. As a result, a defect called a plateless portion tends to be formed during Al plating. Moreover, since Si is also an element which hardens a steel sheet, the Si content of the steel sheet in the present invention which is required to have a high formability is preferably as low as up to 0.2%, more preferably up to 0.1%, still more preferably up to 0.03%.
Mn: Although Mn is an element effective in highly strengthening a steel sheet, the present invention is intended to provide a mild steel sheet. The steel sheet is, therefore, preferred to have a lower Mn content. Since the steel is hardened so that the production of a steel sheet having a high ductility becomes difficult when the Mn content exceeds 0.6%, the Mn content is determined to be less than 0.6%, preferably less than 0.4%, more preferably less than 0.3%.
P: P is an element which segregates at grain boundaries to embrittle the grain boundaries, and it is also an element which lowers the ductility of a steel sheet. Accordingly, a lower P content is preferred. Moreover, P markedly influences the air-tightness after welding for reasons which are not understood, and greatly deteriorates the air-tightness after welding of a steel sheet to which even B is added when P is added in an amount exceeding 0.04%. Accordingly, the P content is restricted to up to 0.04% in the present invention. In order to obtain the air-tightness after welding more stably, the P content is preferably up to 0.02%, more preferably up to 0.01%.
N: A lower N content is preferred for reasons as mentioned in the C content. From the standpoint of ensuring the formability, the upper limit of the N content is determined to be 0.01%, preferably up to 0.006%.
Ti, Nb: The elements are known to fix C and N. A steel sheet which substantially contains neither solute C nor solute N as a result of fixing C and N with the elements is known as an IF steel. Such an IF steel is naturally mild, and excellent in deep drawability. Ti is added for the purpose as mentioned above also in the present invention. The addition amount is preferably at least the atomic equivalent of (C+N). However, when the contents of C and N are very small, the Ti content may be the magnitude of the content of impurities. Accordingly, the lower limit is not determined specifically. When the addition amount is excessive, the effect is saturated. Moreover, since Ti is an element which promotes an Alxe2x80x94Fe reaction, the alloy layer is likely to become thick when the content is large, and tends to lower the formability of the steel sheet. The upper limit is, therefore, determined to be 0.2%. Since Nb is an element which raises the recrystallization temperature, Ti is preferred to be used in combination.
Al: Al is similar to Si in that it has strong affinity with O, and tends to make hot-dip Al plating difficult. Moreover, since Al forms Al2O3 inclusions to lower the formability of a steel sheet, the content of Al is determined to be up to 0.1% as acid-soluble Al. Although the lower limit is not determined specifically, addition of Al to some extent is preferred because it inhibits formation of surface defects caused by Ti oxides. A preferred addition range is from 0.01 to 0.05%.
B: B is an element important in ensuring the air-tightness after welding in the present invention. B has been known to improve the secondary formability, at the time of suffering outer force subsequent to deep drawing once, and the fatigue strength. The present inventors have further found that the grain structure in the weld zone subsequent to Al plating is modified so that the air-tightness of the weld zone is greatly improved. In order to obtain such effects, B must be added in an amount of at least 0.0001%. Moreover, the B addition naturally exerts effects on the secondary formability and fatigue strength. In order to obtain stabilized properties, addition of B in an amount of at least 0.0003% (3 ppm) is desired. However, when the addition amount becomes excessive, the high temperature strength becomes overly high, and the capability of being hot rolled decreases. Accordingly, the upper limit is determined to be 0.0030%.
Cr: Cr is an element which increases the potential of the steel sheet. Addition of the element can decrease the potential difference between the alloy layer and the steel sheet substrate. Cr in an amount of at least 0.5% is necessary for achieving the effects. Moreover, when the Cr content exceeds 7%, the surface enrichment of Cr oxides becomes significant in the hot-dip plating step, and plating becomes difficult in a conventional process. Accordingly, the above-mentioned value is determined to be the upper limit.
Cu, Ni, Mo: These elements can be added if necessary. Cu, Ni and Mo are elements which contribute to the improvement of the corrosion resistance of the steel sheet. Ni and Mo particularly improve the pitting corrosion resistance. In order to manifest these effects, Cu, Ni and Mo must be added in an amount of at least 0.05%. On the other hand, excessive addition of Cu may cause the formation of scabs during hot rolling. Since the effects of adding Ni and Mo are saturated even when they are added excessively, the upper limit content is determined to be 0.5% (Cu, Ni, Mo)
Next, reasons for the restriction of the plating layer will be explained. The addition amount of Si in the plating layer will be explained. The element is usually added in an amount of approximately 10% usually for the purpose of thinning the alloy layer. As described above, the alloy layer formed during hot-dip Al plating is very hard and brittle. The alloy layer, therefore, tends to become the starting point of destruction, and reduces the ductility of the steel sheet itself. Even an ordinary alloy layer having a thickness of about 2 to 3 xcexcm lowers the ductility of the steel sheet by 2 to 5 points (2 to 5%). Accordingly, when the alloy layer is thinner, it acts more advantageously toward forming. The effects of adding Si on decreasing the alloy layer thickness is not significant unless it is not added in an amount of at least 2%. Moreover, when the addition amount exceeds 13%, Si tends to become electrochemically cathodic in addition to the saturation of the addition effects. Accordingly, an increase in the Si content results in deterioration of the corrosion resistance of the plating layer. The Si content is, therefore, restricted to 2 to 13%.
When the amount of the Al plating increases, the corrosion resistance of the steel sheet increases, whereas the adhesion of the plating and the weldability tend to be deteriorated. The fuel tank material for automobiles which must be subjected to severe forming and welding is preferred to be plated in an amount of up to 50 g/m2 per side. On the other hand, since a thick alloy layer exerts adverse effects on the ductility of the Al-plated steel sheet as described above, a thinner alloy layer is more preferred.
In the present invention, it has been discovered that improvement of the ductility of the steel sheet is effective in inhibiting crack formation in the Al plating layer. When the steel sheet has a total elongation of at least 45% after Al plating, cracks are hardly formed even when the steel sheet is formed severely and, therefore, the corrosion resistance subsequent to forming is also improved. The total elongation is restricted to at least 45% for the reasons mentioned above. Though reasons for inhibiting the crack formation are not definite, stress concentrations of some kind appears to be relaxed. Although a higher upper limit is better, the production of the steel sheet having a total elongation exceeding 60% becomes uneconomical, the total elongation of 60% becomes a practical upper limit.
The steel sheet may be subjected to chromate treatment as a primary rust prevention, temper rolling for adjusting the surface condition and the material quality, resin coating for imparting lubricity, and the like treatment as after treatment of the plating. In the present invention, a chromate coating is preferably imparted to the steel sheet after plating. Any of the known chromates such as an inorganic chromate and an organic chromate may be employed, and any of the known chromate treatments such as a coating procedure and a reaction procedure may be employed. The chromate treatment mainly improves the weldability, and the treatment naturally improves the corrosion resistance in addition to the weldability. The coating amount of the chromate is determined to be from 5 to 100 mg/m2 per side as Cr. The coating amount is decided as mentioned above because the effects on the weldability are not significant 2 when the coating amount is less than 5 mg/m2, and because the effects are saturated when the coating amount is at 2 least 100 mg/m2. Moreover, a resin coating is preferably imparted to the top surface of the steel sheet.
The resin coating contributes to the lubricity, inhibition of a reaction between an electrode and the steel sheet during resistance welding, and the like, improves the properties such as formability and weldability, and comprehensively gives the steel sheet excellent properties for fuel tanks. When the thickness of the organic coating is slight, the steel sheet may be directly coated with the organic coating, or a chromate may be added to the organic coating.
The potential difference between the alloy layer and the steel sheet substrate for plating is determined to be up to 0.35 V. The measurement environment is preferably one containing formic acid so that a corrosion environment close to that within actual fuel tanks is formed. Conventional Al-plated steel sheets show a potential difference of about 0.4 V in such an environment. I-Corrosion tends to proceed between the alloy layer and the steel sheet substrate for plating in such an environment as described above. When the potential difference is small, the corrosion proceeds only slightly in an Al-plated steel sheet which even has cracks in the plating layer and the alloy layer. When the potential difference is within the range, it does not matter whether the alloy layer is nobler than the steel sheet substrate or vice versa. However, it does not appear that the alloy layer actually often becomes baser.
Next, reasons for restricting addition elements in the Al plating layer will be explained. The plating is an Alxe2x80x94Si type one, and Sn, Zn, Sb and Bi can be added in a total amount of 0.5 to 5%. All these elements lower the potential of the alloy layer when mixed therein. The effects are manifested when the elements are added in a total amount of at least 0.5%. Since excessive addition of the elements deteriorates the corrosion resistance of the plating layer, the upper limit is determined to be 5%.
The steel sheet is produced by a conventional process. The molten steel is prepared by adjusting the chemical composition using, for example, converter-vacuum degassing treatment, and the slab is produced by continuous casting, or the like process, followed by hot rolling the slab. The conditions of the hot rolling and cold rolling subsequent thereto influence the deep drawability of the steel sheet. In order to impart a particularly excellent deep drawability to the steel sheet, the following procedures are recommended: the heating temperature of the steel at the time of hot rolling is as low as about 1,150xc2x0 C.; the finish temperature of hot rolling is as low as about 800xc2x0 C.; the coiling temperature is as high as at least 600xc2x0 C.; and the reduction of the cold rolling is as high as about 80%.